Sundbyfoss is a village in Holmestrand Municipality in Vestfold county, Norway. The village is located along the lake Hillestadvannet, about 3 km south of the village of Hof. The village of Gullhaug lies about 7 km to the southeast and the town of Holmestrand lies about 10 km to the southeast. The village of Hvittingfoss in Kongsberg Municipality lies about 8 km to the west. Hillestad Church lies about 3 km east of the village. The 0.47 km2 village has a population (2024) of 562 and a population density of 1195 PD/km2. Sundbyfoss is named after the local waterfall on the river Sundbyelva. The waterfall provided the basis for early industry in the village. Today, Sundbyfoss is mainly a residential area. The largest industrial enterprise is the "Haslestad Bruk" sawmill. Historically, the old private Tønsberg–Eidsfoss railway line and Holmestrand–Vittingfoss railway line both went through Sundbyfoss.